Surface acoustic wave filter and multistage filter with multiple inside busbar lead-out electrodes

ABSTRACT

A surface acoustic wave resonator is constituted by an IDT electrode and reflector electrodes disposed on both sides thereof, on a piezoelectric substrate. Two of said surface acoustic wave resonators are disposed nearby so that the propagation directions of the respective surface acoustic waves are in parallel to each other to make acoustic couple to constitute a surface acoustic wave filter having plural exciting modes with different propagation frequencies. The bus bar electrodes of each of two IDT electrodes are electrically separated from each other, and the leading out electrodes led out from at least two spots on those bus bar electrodes are electrically connected to each other, by which one side of the balanced input and output terminal. As a result, the electrode resistance of the IDT electrode is alleviated to make the insertion loss less.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a surface acoustic wave filter to beused for a high frequency wave circuit or the like in, for example, aradiocommunication apparatus.

2. Related Art of the Invention

The electromechanical function parts using surface acoustic wave (SAW)have been noted with current attention in the hardware high densitybecause the acoustic velocity of the wave is several kilometers/second,and the wave energy has properties to be concentrated on the surface ofthe propagation medium. Due to the development of the interdigitaltransducer (IDT) electrode and progress of the thin film preparingtechnique which has made its modified development possible, the same hasbeen practically utilized for delay line for radar, band-pass filtersfor television receiver, etc. At present, the SAW is extensively used asRF and IF stage filters for receiving and transmitting circuits inradiocommunication apparatus.

In recent years, in consequence of the adoption of digital systems formoving objects, development of digital portable telephone and digitalcordless telephone is intense. As the communication systems to be usedfor these apparatuses have information on the amplitude and phase ofsignals, the flatness of amplitude characteristics and group delaydeflection characteristics are required for the filters to be used forIF stage. Also, as the excellent characteristic is required for theselectivity to distinguish the signal of an adjacent channel and thedesired signal an acute attenuation characteristic having narrowtransition bandwidth is also an essential condition. Also, recently,balanced input and output of the IC device in the stage before and afterthe IF filter have progressed, and the balanced input and output arerequired for the IF filter.

Conventionally, as a SAW filter suitable for the IF stage there areknown a transversal SAW filter and two kinds of longitudinal modecoupled and transverse mode coupled type SAW filters. The transversalSAW filter has an excellent group delay deflection characteristic, butit has large insertion loss, poor attenuation characteristic, and largeelement size. On the other hand, the mode coupled type SAW filterpresents an acute attenuation characteristic, shows small insertionloss, and is small in element size, but its group delay deflectioncharacteristic is inferior to that of a transversal type SAW filter. Thelongitudinal mode type SAW is characterized by having a relatively largespurious zone on the high band side in the vicinity of the passing band,and the transverse mode type SAW filter is characterized by having avery narrow band characteristic. In view of the above characteristics,as the IF filter for the mobile communication apparatus the transversemode coupled type SAW filter which is miniature in size and excellent inattenuation characteristic has been widely used.

Hereinafter, explanation is made on the conventional transverse modecoupled type SAW filter.

FIG. 24 is a constitution view showing a transverse mode coupledresonator type SAW filter according to conventional technique. In FIG.24, the part 241 is a single crystal piezoelectric substrate. By formingan electrode pattern on the piezoelectric substrate 241, the SAW can beexcited. The part 242a is an IDT electrode formed on the piezoelectricsubstrate 241, and by setting the reflector electrodes 242b and 242c onboth sides thereof, an energy sealing in type SAW resonator is formed.On the piezoelectric substrate 241, there is formed a similar SAWresonator by the IDT electrode 243a and the reflector electrodes 243band 243c. And, these two resonators are disposed nearby, and because ofthe formation of an acoustic couple between them, the SAW filter isconstituted.

In the SAW filter constituted as above, two kinds of SAW modefrequencies to be excited on the piezoelectric substrate are determinedby the electrode finger crossing width of IDT electrodes and thedistance between the two SAW resonators disposed nearby, and the passingband width of the filter is determined.

In the SAW filter constituted as above, the bandwidth that can berealized is very narrow, and the specific bandwidth of the filter to berealized (normalized bandwidth at the central frequency of the filter)is at most about 0.1%. In order to meet the recent digital system, it isrequired to make the filter passing characteristics of wider bandwidthand broaden the flat bandwidth of the group delay deflectioncharacteristic.

Also, recently a balanced input and output of IC device in the pre- andpost-stages of IF filter have progressed. Accordingly, a balanced inputand output type is strongly demanded for the IF filters. However, asshown in FIG. 24, in the conventional SAW filters, the one side of theelectrode fingers of input and output stages of the IDT electrodes 242a,243a is grounded, and there is a problem that the filter cannot beformed in a balanced input and output type.

Furthermore, there has been desired the impedance matching between theIF filter and the IC devices in the pre- and post-stages thereof, and asthe input and output impedance's of the conventional filters depend onthe number of pairs of the electrode fingers included in the IDTelectrodes which are closely related with the filter characteristic,there has been a problem of it being difficult to obtain the desiredimpedance value simultaneously with obtaining the desired filtercharacteristic.

SUMMARY OF THE INVENTION

The present invention is to settle the above problems in the prior art,and its objects are (1) to realize a balanced type input and outputconstitution and to improve balancing extent of the balanced typeterminal in the input and output terminal and realize low insertionloss, (2) to make the pass band wide width, and to make the phase andamplitude characteristics flat, and (3) to provide a SAW filter havingthe desired input and output impedances.

In order to attain the above objects, the SAW filter of the presentinvention comprises first and second surface acoustic wave resonatorseach having a reflector electrode on both sides of an IDT electrode asan inter-digital transducer electrode, said resonators being disposednearby in positions in which directions of propagation of the respectivesurface acoustic waves are parallel with each other and acousticallycoupled,

an inside bus bar electrode included in the first IDT electrode of thefirst surface acoustic wave resonator and an inside bus bar electrodeincluded in the second IDT electrode of the second surface acoustic waveresonator being mutually electrically separated,

said first IDT electrode being connected to a balanced type inputterminal, and said second IDT electrode being connected to a balancedtype output terminal,

one terminal of said balanced type input terminal being electricallyconnected to leading out electrodes led out directly or indirectly fromat least two places of the inside bus bar electrode of said first IDTelectrode, and one terminal of said balanced type output terminal beingelectrically connected to leading out electrodes led out directly orindirectly from at least two places of the inside bus bar electrode ofsaid second IDT electrode, thereby performing balanced operation.

By this constitution, there can be obtained, for example, a basicelectrode pattern of SAW filter having a balanced type input and outputterminal having low insertion loss and favorable balancing level.

Also, in order to settle the problems mentioned above, the SAW filter ofthe present invention comprises first and third surface acoustic waveresonators each having a reflector electrode on both sides of an IDTelectrode as an inter-digital transducer electrode, said resonatorsbeing disposed on a piezoelectric substrate in positions in whichdirections of propagation of the respective surface acoustic waves areparallel with each other,

a plurality of strip line electrodes being disposed in parallel betweensaid first and third surface acoustic wave resonators in the sameelectrode period as those of the first and third surface acoustic waveresonators, said plural strip line electrodes being connected oneanother by bus bar electrodes to form a second surface acoustic waveresonator having periodic structured electrode rows, said first andthird surface acoustic wave resonators being disposed nearby to saidsecond surface acoustic wave resonator to make acoustic couple, and theadjacent bus bar electrodes between said surface acoustic waveresonators being electrically separated, and all periodic structuredelectrodes of said second surface acoustic wave resonators beinggrounded,

assuming that an electrode finger crossing width of IDT electrodesconstituting the first and third surface acoustic wave resonators to beW1, and a strip line length of said periodic structured electrode rowsconstituting the second surface acoustic wave resonator to be W2, therelative size of W1 to W2 being set to 1≦W2/W1.

By this constitution, the distance between the three resonancefrequencies becomes equal, and when the input and output coordination isobtained, the ripples in the pass band decrease to give excellent passcharacteristics. As a result, there can be obtained the SAW filterhaving broad bandwidth and flat pass characteristics and acuteattenuation characteristics.

Furthermore, in order to solve the above problems, the SAW filter of thepresent invention comprises at least two surface acoustic waveresonators each having a reflector electrode on both sides of an IDTelectrode as an inter-digital transducer electrode, at least two of saidresonators being disposed on a piezoelectric substrate in positionsnearby to one another in which directions of propagation of therespective surface acoustic waves are parallel with one another to makeacoustic couple,

characterized in that, of plural electrode fingers included in at leastone IDT electrode, at least a couple of adjacent electrode fingers arein reverse phase relations to each other, and said plural electrodefingers are connected so as not to cancel the respective electriccharges.

By this constitution, there can be obtained an SAW filter having thedesired input and output impedance.

As described above, according to this invention, it is possible toprovide a compact SAW filter which shows the smaller insertion loss thanthe conventional one, improved balancing level in the balanced typeinput and output terminal, or, which can be provided with flat filterpass characteristic and good extra-band attenuation characteristic, orwhich has the desired input and output impedance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a constitution view showing a SAW filter according to thefirst embodiment of the present invention.

FIG. 2 is a constitution view showing another example of SAW filteraccording to the first embodiment of the present invention.

FIG. 3 is a constitution view showing a multi-stage SAW filter accordingto the first embodiment of the present invention.

FIG. 4 is a constitution view showing another example of a multi-stageSAW filter according to the first embodiment of the present invention.

FIG. 5 is a constitution view showing a SAW filter according to thesecond embodiment of the present invention.

FIG. 6 is a constitution view showing another example of a SAW filteraccording to the second embodiment of the present invention.

FIG. 7 is a constitution view showing another example of a SAW filteraccording to the second embodiment of the present invention.

FIG. 8 is a constitution view showing a multi-stage SAW filter accordingto the second embodiment of the present invention.

FIG. 9 is a constitution view showing another example of a multi-stageSAW filter according to the second embodiment of the present invention.

FIG. 10 is a constitution view showing a SAW filter according to thethird embodiment of the present invention.

FIG. 11 is a distribution chart of an excitation mode for illustratingthe operation of the SAW filter according to the third embodiment of thepresent invention.

FIG. 12 is a characteristic chart of the resonance frequency of eachmode to the value of W specified by the SAW wavelength λ in the case ofW1=W2=W in the third embodiment of the present invention.

FIG. 13 is a representative actual measurement chart showing acomparative example of the passing characteristics of the SAW filter inthe third embodiment of the present invention.

FIG. 14 is an actual measurement view of a resonance mode frequencydifference to W2/W1 in the third embodiment of the present invention.

FIG. 15 is an actual measurement view showing the pass characteristic ofSAW filter in the third embodiment of the present invention.

FIG. 16 is a constitution view showing another example of the SAW filterin the third embodiment of the present invention.

FIG. 17 is a constitution view showing a SAW filter in the fourthembodiment of the present invention.

FIG. 18 is a constitution view showing a SAW filter in the fifthembodiment of the present invention.

FIG. 19 is a capacity equivalent circuit diagram of the SAW filter inthe fifth embodiment of the present invention.

FIG. 20 is a constitution view showing another example of SAW filter inthe fifth embodiment of the present invention.

FIG. 21 is a constitution view showing another example of SAW filter inthe sixth embodiment of the present invention.

FIG. 22 is a constitution view showing another example of SAW filter inthe seventh embodiment of the present invention.

FIG. 23 is a constitution view showing another example of SAW filter inthe eighth embodiment of the present invention.

FIG. 24 is an electrode pattern diagram of conventional SAW filter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will beillustrated with reference to the drawings.

(Embodiment 1)

FIG. 1 is a constitution view showing a SAW filter according to thefirst embodiment of the present invention. By forming an electrodepattern having a strip line shaped periodical structure on a singlecrystal piezoelectric substrate 11 shown in FIG. 1, SAW can be excited.On the piezoelectric substrate 11 there is formed a first SAW resonatorof energy stage type constituted by an IDT electrode 12a and reflectorelectrodes 12b, 12c. Also, on the piezoelectric substrate 11, there isconstituted a second SAW resonator of constituted by an IDT electrode13a and reflector electrodes 13b, 13c. And, these two SAW resonators aredisposed close to each other, and by formation of acoustic couplebetween them, a SAW filter is constituted.

A remarkable difference in the electrode pattern constitutions betweenthe SAW filter of the first embodiment of the present invention shown inFIG. 1 and that of prior art as shown in FIG. 24 is that the bus barelectrodes 244 common to the two resonators disposed nearby ofconventional style in FIG. 24 are electrically separated in the IDTelectrode part into the inside first bus bar 14 and the second bus bar15 in the first embodiment of the present invention. The first bus bar14 belongs to the first SAW resonator, and the second bus bar 15 to thesecond SAW resonator. By this bus bar separation constitution, the firstand second SAW resonators can have fully electrically independent inputor output stages. Namely, the balanced input stage of the first SAWresonator is constituted by an electrode finger formed by being bound bythe first bus bar electrode 14 and an IDT electrode 12a comprising anelectrode finger to be coupled with said electrode finger. In the samemanner, the balanced output stage of the second SAW resonator isconstituted by an IDT electrode 13a comprising an electrode fingerformed by being bound by the second bus bar electrode 15 and anelectrode finger to be coupled with said electrode finger. Here, thefirst IDT electrode of the present invention corresponds to the IDTelectrode 12a. The second IDT electrode of the present inventioncorresponds to the IDT electrode 13a.

The connection of the signal line to the balanced circuit constituted asabove may be made to apply an input signal to the spot between the firstbus bar electrode 14 and the third bus bar electrode 14a positionedoutside the IDT electrode to be coupled therewith, and to take out theoutput signal from the spot between the second bus bar electrode 15 andthe fourth bus bar electrode 15a positioned outside the IDT electrode tobe coupled therewith. By this step, the object of balancing the inputand output terminals has been attained. When this is observed from theaspect of the insertion loss, the amount is about 3.2 dB in the case ofthe above connection.

With respect to one terminal of the balanced type input terminaldescribed above, a connection line is led from one spot of the first busbar electrode 14, and as to the one terminal in the balanced type outputterminals, explanation has been given on the case where a connectionline is led from one spot of the second bus bar electrode 15. Againstthis, the case of the constitution leading out the connection lines fromthe two spots of the first and second bus bar electrodes 14, 15 isdescribed next.

With respect to this insertion loss, by leading out two connection lines(leading out electrode fingers 16a, 16b) from two spots of the first busbar electrode 14 to make a terminal of the input side, and leading outtwo connection lines (leading out electrode fingers 17a, 17b) from twospots of the second bus bar electrode 15 to make a terminal of theoutput side, improvement of the balancing level in the balanced typeinput and output terminal is realized, the difference of loss formed ineach terminal is decreased to reduce the above insertion loss to a largeextent to about 2.8 dB. This is an effect having an important value in aminiature type portable communication apparatus which weighs as beingimportant in the minor loss and a balance level in the balanced typeinput and output terminal. In other words, in FIG. 1, from both ends ofthe first bus bar electrode 14, the leading out electrode fingers 16a,16b directed outward are formed on the space between the IDT electrode12a and the reflector electrodes 12b, 12c, and by connecting the endparts of these electrode fingers as illustrated, the effect as mentionedabove is obtained. The leading out electrode fingers 17a, 17b at bothends of the second bus bar electrode 15 also have the same effect. Theleading out electrode fingers 16a, 16b can be regarded as beingconstituted by the electrode fingers having the same length as otherelectrode fingers which are connected to the two end parts of the firstbus bar electrode 14 and the leading out electrodes of short lengthconnected to the front end parts of those two electrode fingers. Thesame comments can be made on the leading out electrode fingers 17a, 17b.

FIG. 2 shows an example of variations of the first embodiment of thepresent invention shown in FIG. 1. To the parts which perform the samefunctions as those given in FIG. 1 the same marks are affixed andexplanations thereon are omitted.

The wiring pattern 21 to connect between the leading electrode fingers16a and 16b is formed on the piezoelectric substrate 11 and has a linewidth wider than the resonator electrode. A part of it is furtherexpanded as shown in FIG. 2 to form a one connection land 21a forconnecting between the balanced type input and output terminals and theoutside wiring member 25a.

The wiring pattern 22 for connecting between the leading out electrodefingers 17a and 17b is formed on the piezoelectric substrate 11, and hasa line width wider than the resonator electrode width. A part of it isfurther expanded, as shown in the same figure, to form one connectionland 22a for the connection line between the balanced type outputterminal and the outside wiring member 26a.

The bus bar electrode 14a is extended outward to form another connectionland 23 for connecting between the balanced type output terminal and theoutside wiring member 25b. The bus bar electrode 15a is also extendedoutward to form another connection land 24 for connecting between thebalanced type output terminal and the outside wiring member 26b.

The above constitution is effective for assuring the characteristics oflow insertion loss and good balanced level of the SAW filter having lowinsertion loss and balanced type input and output terminals, and forstabilizing the filter characteristics.

Taking an example of a SAW filter of single stage constitution,explanation has been given above by referring to FIG. 1 and FIG. 2. SuchSAW filter can be used in multi-stage constitution.

FIG. 3 is an example thereof, and when a multi-stage connection SAWfilter is constituted by connecting a plurality of SAW filters on thesame piezoelectric substrate 31, great improvement can be obtained inthe characteristics in rejection band and transition band, though theremay be some increase in the insertion loss. The two-stage verticallyconnected filters shown in FIG. 3 comprise a first SAW resonatorconstituted by an IDT electrode 12a and reflector electrodes 12b, 12c asexplained in FIG. 1, and a second SAW resonator constituted by an IDTelectrode 13a and reflector electrodes 13b, 13c, which are disposed nearto each other to form a SAW filter 32 and a SAW filter 33 of the sameconstitution thereof formed on the piezoelectric substrate 31, and thetwo members are connected by a connecting wire. A bus bar electrode 14ais connected with one balanced input terminal via external wiring 34,and leading out electrodes 16a and 16b are connected with the otherbalanced input terminal via external wiring 35. A bus bar electrode 15bis connected with one balanced output terminal via external wiring 36,and leading out electrodes 17a and 17b are connected with the otherbalanced output terminal via external wiring 37.

In FIG. 3, the leading out electrodes 17a and 17b on the output side ofthe first stage SAW filter 32 are connected to the leading outelectrodes 16a and 16b on the input side of the next stage SAW filter 33with the connecting wires 39a and 39b, respectively. The bus barelectrode 15a of IDT electrode which is another output of the firststage output is connected by the connecting wire 40 to the IDT electrode14a which is another output of the next stage.

In this manner, even between the filter stages, there can be realizedreduction of increase in insertion loss at the time of the multi-stageoperation and improvement to the balance level of balanced type inputand output terminals, by connecting one part of the IDT electrodes attwo places of 39a and 39b.

The wire connections of the multi-stage filter on the input side andoutput side as shown in FIG. 3 are similar to those of FIG. 1, and havethe same action and effect.

FIG. 4 shows an example where the inter-stage and input and outputwirings are carried out by the wiring patterns formed on the substrate41.

On the piezoelectric substrate 41, there are formed the first SAW filter42 and the second SAW filter 43 which have the same constitutions as theSAW filters shown in FIG. 1, FIG. 2, and FIG. 3.

The leading out electrodes 17a and 17b on the output side of the firstSAW filter 42 are connected to the leading out electrodes 16a and 16b onthe input side of the second filter 43 by forming the first inter-stageconnection electrodes 44a, 44b of wider width than the resonatorelectrodes on a piezoelectric substrate 41. Also, another output 15a ofthe first filter 42 and another input 14a of the second filter 43 areconnected by forming the second inter-stage connecting electrode 45having wider width than the electrode of the resonator on thepiezoelectric substrate 41.

The leading out electrodes 16a and 16b on the input side of the firstfilter 42 are connected by the wiring pattern 46 having the wider widththan the resonator electrode formed on the piezoelectric substrate 41.Further, a part of said wiring pattern 46 is further expanded to formone connecting land 46a with the outer wiring member 47a of the balancedtype input terminal, and the bus bar electrode 14a of outside IDTelectrode is expanded outward to form a connecting land 48 with theexternal wiring member 47b of the balanced type input terminal.

On the other hand, the area between the leading out electrodes 17a and17b on the output side of the second filter is connected by the wiringpattern 46b of wider line width than the resonator electrode widthformed on the piezoelectric substrate 41. Further, a part of the saidwiring pattern is further extended to form a connection land 46c withthe outside wiring member 47c of the balanced type output terminal, andthe bus bar electrode 15a is extended outward to form a connection land48a with the external wiring member 47d of the balanced type outputterminal.

By such a pattern constitution, there can be provided a balanced typemulti-stage SAW filter having low insertion loss and good balancinglevel.

The lands 44c, 45a for external wiring provided on the inter-stageconnection electrodes 44b, 45 of FIG. 4 are useful in the connection ofthe external circuit element for filter characteristic adjustment.

By the way, there may be cases where the desired good transmissioncharacteristics cannot be obtained because of the mismatching of theinput and output impedances in the stages.

In such a case, the reactance element such as an inductor may beconnected as a matching element to the inter-stage connecting electrodeto make adjustment. The lands 44c, 45a for external wiring are usefulfor the purpose. Alternatively, by adopting such a constitution that areactance element such as a spiral inductance is formed on the samepiezoelectric substrate 41 or on a separate substrate and connected tothe inter-stage connecting electrode, no extra space is necessitated,and reduction of filter circuit size can be easily realized. Thereactance element for adjustment may be connected to either one of theinter-stage connecting lands 44c, 45a and the other land may begrounded. According to the experiment, improvements of the symmetricproperty of the filter transmission characteristic is observed in thecase that the reactance element is connected to the first connectionland 44c.

(Embodiment 2)

FIG. 5 is a constitution view showing a SAW filter according to thesecond embodiment of the present invention.

By forming an electrode pattern having a strip line shaped periodicalstructure on a piezoelectric substrate 51 shown in FIG. 5, SAW can beexcited. On the piezoelectric substrate 51 there is formed a first SAWresonator of energy strage type constituted by an IDT electrode 52a andreflector electrodes 52b, 52c. Also, on the piezoelectric substrate 51,there is constituted a third SAW resonator constituted by an IDTelectrode 54a and reflector electrodes 54b, 54c.

The point to be specially noted here is that the IDT electrode part ofthe second SAW resonator formed between the first SAW resonator and thethird SAW resonator accompanied with the reflector electrodes 53b, 53c,has a similar structure to that of the reflector electrode, and isconstituted by a periodic structure strip line electrode row 53a havingapproximately the same length as the crossing width of the electrodefingers of the IDT electrodes 51a, 54a in the first and third SAWresonators.

In other words, even if the structure of the electrode part of thesecond SAW resonator is not of the same structure as those of theabove-described IDT electrodes 52a, 54a but is changed to the periodicstructured strip line electrode row 53a, if the electrode period is thesame, the SAW can be transmitted in entirely the same manner.Accordingly, the acoustic behaviors of the central part second SAWresonator make no difference from the case of the IDT electrodestructure.

The above three SAW resonators have the acoustic couple closely disposedto one another. The bus bar electrodes of the parts adjacent to oneanother are electrically independent. From both ends of the bus barelectrode 55 adjacent to the second SAW of the IDT electrode in thefirst SAW resonator, there are formed outward the first and secondelectrode fingers 57a and 57b which constitute a part of the balancedtype input terminal, in the space between the IDT electrode 52a and thereflector electrodes 52b, 52c. Also, from both ends of the bus barelectrode 56 adjacent to the second SAW of the IDT electrode in thethird SAW resonator, there are formed outward the third and fourthelectrode fingers 58a and 58b which constitute a part of the balancedtype output terminal, in the space between the IDT electrode 54a and thereflector electrodes 54b, 54c. The electrode constitutions describedabove are the basic constitutions of the triple mode SAW filter havingthe balanced type input and output terminals of low insertion lossaccording to the present invention.

FIG. 6 shows an example of connection of a balanced type input andoutput terminal of the present invention for the triple mode SAW filteras explained in FIG. 5.

As shown in said figure, the first electrode finger 57a and the secondelectrode finger 57b of the first SAW resonator are connected by theconnecting wires 61a, 61b to make one input terminal of the balancedtype input terminal, and the connecting wire 62 is led out from the busbar electrode 55a of the outside IDT electrode to make the other inputterminal of the balanced type input terminal. And, the third electrodefinger 58a and the fourth electrode finger 58b of the third SAWresonator are connected by the connecting wires 63a, 63b to make oneoutput terminal of the balanced type input terminal, and the connectingwire 64 is led out from the bus bar electrode 56a of the outside IDTelectrode to make the other output terminal of the balanced type inputterminal.

FIG. 7 shows another embodiment of the constitution of the balanced typeinput and output terminal of the triple mode SAW filter.

As shown in said figure, the area between the first electrode finger 57aof the first SAW resonator and the second electrode finger 57b isconnected by the wiring pattern 71 of wider line width than theresonator electrode width formed on the piezoelectric substrate 51.Further, the pattern 71 is further extended to form a connection land71a with the external wiring member 75a, and the bus bar electrode 55aof IDT electrode is extended outward to form a connection land 73 withthe external wiring member 75, and the area between the third and fourthelectrode fingers 58a and 58b of the third SAW resonator is formed on apiezoelectric substrate 51 to make a resonator electrode, and connectionis made by the wiring pattern 72 which has the wider line width than theresonator electrode. The pattern 72 is further extended to form aconnection land 72a with the external wiring member 76a, and the bus barelectrode 56a of IDT electrode is extended outward to form a connectionland 74 with the external wiring member 76. According to such aconstitution, similarly to what SAW described in the first embodiment,it becomes possible to provide a triple mode SAW filter in which theinsertion loss is further reduced and connection with the externalcircuit is easy, as explained in the first embodiment.

FIG. 8 shows an example of the case where a plurality of the triple modeSAW filters as explained with reference to FIG. 5 are stepwise connectedvertically.

As shown in the figure, on the piezoelectric substrate 81 there areformed a first triple mode SAW filter 82 and a second triple mode SAWfilter 83. The third and fourth electrode fingers 58a, 58b on the outputside of the first filter 82 and the bus bar electrode 56a on the outputside are stepwise connected to the first and second electrode fingers57a, 57b on the input side and the bus bar electrode 55a on the inputside, of the second filter 83, by the connecting wires 83a, 83b, and 84.The parallel type wire connections of the input circuit and outputcircuit are entirely same as the wiring constitution of the single stagefilter shown in FIG. 6.

FIG. 9 shows another example of the input and output constitutions andthe inter-stage constitutions of the vertical connection triple mode SAWfilter as shown in FIG. 8.

As shown in said figure, on the piezoelectric substrate 91, there areformed a first triple mode SAW filter 92 and a second triple mode SAWfilter 93. The two filters are Inter-stage connected by the inter-stageconnecting electrodes 94a, 94b, and 95 having wider widths than thewidth of the resonator electrode which is formed by placing the thirdand fourth electrode fingers 58a, 58b on the output side, and the busbar electrode 56a, of the first filter 92, and the first and secondelectrode fingers 57a, 57b on the input side, and the bus bar electrode55a on the input side of the second filter 93, on the piezoelectricsubstrate 91. The lands 94c, 95a formed on a part of each connectingelectrode are convenient to use for the connection of the externalelements for adjusting filter characteristics. The wiring patterns ofthe input circuit and output circuit are entirely same as those of thesingle stage filter constitution shown in FIG. 7.

As described above, according to the embodiments 1 and 2, because thebus bar electrode of the IDT electrode is electrically independent,balanced input and output mode can be realized, and accordingly, thefilter characteristics do not have the effects of floating capacity bythe grounding condition of electrode, so that the characteristics in therejection band and transition band are improved, and moreover, due tothe leading out electrode structure which is characterized by thepresent invention, remarkable improvement of insertion loss andimprovement in balance level in the balanced type input and outputterminal can be realized.

In the embodiment 3, there is employed an example wherein, as a balancedtype triple mode filter, there is used one in which the IDT electrode ofthe central part resonator as shown in FIG. 5 has a periodic structuredelectrode constitution same as the reflector electrode. Even when thispart is an IDT electrode structure same as being heretofore used, theeffect of improvement in the filter characteristic by the balancedwiring connection by the present invention is obtainable in exactly thesame manner.

(Embodiment 3)

FIG. 10 is a constitution view showing the third embodiment of the SAWfilter according to the present invention.

In FIG. 10, the part 101 is a single crystal piezoelectric substrate. Byforming an electrode pattern on the piezoelectric substrate 101, SAW canbe excited. On the piezoelectric substrate 101 there is formed an energystrage type first SAW resonator constituted by an IDT electrode 102a andreflector electrodes 102b, 102c. Also, on the piezoelectric substrate101, there is formed a third SAW resonator constituted by an IDTelectrode 104a and reflector electrodes 104b, 104c. The electrode part103a of the second SAW resonator formed between the first SAW resonatorand the third SAW resonator accompanied with the reflector electrodes103b, 103c has the same structure as that of the reflector electrode.

As reviewed above, even if the structure of the electrode part 103a ofthe second SAW resonator is not of the same structure as those of theabove-described IDT electrodes but is changed to the periodic structuredstrip line electrode row, if the electrode period is the same, the SAWcan be transmitted in entirely the same manner. Accordingly, theacoustic behaviors of the second SAW resonator disposed at the centralpart make no difference from the case of the IDT electrode structure.

Furthermore, assuming that the electrode finger crossing width of IDTelectrodes 102a, 104a in the first and third SAW resonators is W1, andthe length of the strip line constituting the IDT electrode part 103a ofthe second SAW resonator is W2, setting is so made that the relativesize between W1 and W2 becomes: W1≦W2.

The above three SAW resonators have the acoustic couple closely disposedto one another. The electrode finger of the IDT electrode 102a in thefirst SAW resonator is connected to the balanced type input terminal IN,and the electrode finger of the IDT electrode 104a in the third SAWresonator is connected to the balanced type output terminal OUT. Theperiodic structure strip line electrode row 103a in the second SAWresonator is grounded.

Hereinafter, the operation of the SAW filter constituted as above isexplained.

FIG. 11 is an excitation mode distribution chart of the SAW filter inthe present embodiment. To the parts corresponding to those of FIG. 10the same marks are assigned. In FIG. 11, (a) is a constitution view ofthe electrode of the SAW filter shown in FIG. 10. Due to the closelyrelated disposition of the first to third SAW resonators, acousticcouple is formed therebetween, and there are excited the primary,secondary, and tertiary modes having the potentials as shown in FIG.11(b). Here, due to all electrical grounding of the electrode part 103aof the third SAW resonator disposed at the center, the polarity of thesecondary mode potential distribution is reversible at the center, sothat there can be obtained strong excitation strength on the same levelas that of the primary and tertiary modes. As this permits to constitutea multi-stage mode filter made by effective utilization of the threeexcitation modes, there can be realized a SAW filter having broadbandwidth with acute attenuation characteristics. FIG. 12 shows a changeof the resonant frequency of each mode to the value of W standardized bythe SAW wavelength λ in the case of W1=W2=W, obtained by the wave guidepath mode analysis. The curves 121, 122, and 123 show the changes of theresonance frequencies in primary, secondary, and tertiary modes,respectively. As shown in FIG. 12, to a certain given value W, thefrequency difference Δ1 between the primary mode and the secondary modeand the frequency difference Δ2 between the secondary mode and thetertiary mode become the difference values. Namely, when viewed with 50Ωsystem, as shown in FIG. 13, the pass characteristic of the SAW filterdoes not show equal distance between the peaks of the three resonancemodes as in the curve 131. Accordingly, even when the input and outputare matched, ripples remain in the band as in the curve 132, and thefilter characteristic is degraded.

Here, an effect of the case where the ratio of the length of the stripline W2 constituting the electrode part 103a of the second SAW resonatorto the electrode finger cross difference width W1 of IDT electrodes102a, 104a in the first and third SAW resonator (W2/W1) is shown in FIG.14. In FIG. 14, there is shown a standardized value of the actuallymeasured amount of the frequency difference (Δ1, Δ2 in FIG. 13) inresonance mode to W2/W1 in the SAW filter of the present inventionhaving the constitution of FIG. 10. FIG. 14 shows the values where thelength W2 of the strip line constituting the electrode part 103a of thesecond SAW resonator is varied in the case where the IDT electrodefinger crossing difference width W1 of the first and third SAWresonators is 6.5 wavelength, and the combined gap length G is 1wavelength. As shown in FIG. 14, when the value of W2/W1 is about 1.13,the relation becomes:Δ1=Δ2, i.e., the distance between the threeresonance frequencies becomes equal. As to the allowance range, therelative sizes of W1 and W2 may be set so that they come into the rangeof 1≦W2/W1≦1.3. Practically, considering the scattering in manufacture,the values of W1 and W2 may be set in the range of 1≦W2/W1≦1.16.

FIG. 15 shows the passing characteristic of the SAW filter in he case ofW1=6.5 wavelengths, W2=7.5 wavelengths, i.e., W2/W1=1.15. In FIG. 15,the numeral 151 shows the characteristic of the case observed in 50Ωsystem, and 152 shows the characteristic of the case of matching taken.It can be seen that, in comparison with the case of FIG. 13, the ripplesin the pass band apparently decrease to give excellent passingcharacteristic.

As described above, according to the embodiment 3 of the presentinvention, three SAW resonators are disposed in adjacent relations withone another, and the electrode part of the central SAW resonator isconstituted by a strip line having slightly longer periodic structurethan the cross difference width of the IDT electrode fingers of thefirst and third SAW resonators, and all of them are grounded. By suchconstitution, there can be obtained a SAW filter having wide bandwidthand flat pass characteristic and acute attenuation characteristic.

Furthermore, due to the electrical isolation of the bus bar at thecentral part of the IDT electrode, it becomes possible to wire the IDTelectrode 102a of the first SAW resonator and the SAW resonator 104a ofthe third SAW resonator all independently, so that the balanced inputand output of the SAW filter can be made. Consequently, the filtercharacteristic becomes free from the effect of the floating capacity orthe like depending on the grounding condition of the electrode, and thecharacteristics of the rejection band and transition band are furtherimproved. In addition, it becomes possible to connect the balanced typeelements such as IC to the front and rear stages of the filter withoutusing any external extra circuit such as Balun, thus improving the noisecharacteristics of the whole circuit.

In FIG. 10, the electrode part 103a of the second SAW resonator isgrounded through the electrode pattern existing in the space between theIDT electrode 104a of the third SAW resonator and the reflectorelectrode 104c, but the constitution is not limited to it; and thegrounding may be made through the reflector electrodes 103b, 103c onboth sides of the electrode part 103a.

In this embodiment 3, explanation is given by taking an example of a SAWfilter of single stage constitution. However, as shown in FIG. 16, whena multi-stage connection type SAW filter is constituted by verticallyconnecting a plurality of SAW filters 162, 163 on the same piezoelectricsubstrate 161, though the insertion loss increases to some extent, thecharacteristics of the rejection band and transition band are remarkablyimproved to give more excellent filter characteristics. In this case, itis preferable for the first SAW resonator electrode of the front stageSAW filter to be connected to the balanced type input terminal, and thethird SAW resonator electrode of the rear stage SAW resonator to beconnected to the balanced type output terminal. This is because thefilter can be easily connected to the peripheral circuit such as abalanced type front end IC, making it unnecessary to secure ground forwiring, so that the stabilized filter characteristics are obtainablewith less effect of floating capacity.

By the way, a simple vertical connection of the SAW filters may not givegood transmission characteristic due to the mismatching of the input andoutput impedances in each stage. In such a case, the reactance elementssuch as inductance may be connected as matching elements to theinter-stage connecting electrode patterns 164, 165. In this case, inorder to make full coordination with the balanced type input and outputcircuit, a matching element is required to be connected between theelectrode patterns 164 and 165. However, in practice, the inter-stageportions have no electrical connection with the input and outputterminals but have acoustic couple only. Accordingly, if one electrodepattern (e.g., electrode pattern 165) is directly grounded, and theother electrode pattern (e.g., electrode pattern 164) is groundedthrough the reactance element, the operation similar to the case of areactance element having been connected between the two can be realized.And, when such a constitution is adopted, the wiring for grounding canbe made on an electrode pattern, and therefore the use of bonding wirescan be reduced.

(Embodiment 4)

FIG. 17 is a constitution view showing the fourth embodiment of the SAWfilter according to the present invention.

In FIG. 17, the part 171 shows a single crystal piezoelectric substrate.By forming an electrode pattern on the piezoelectric substrate 171, aSAW can be excited in the same manner as in the third embodiment. On thepiezoelectric substrate 171, there is formed a first SAW resonator ofenergy strage type constituted by an IDT electrode 172a and reflectorelectrodes 172b, 172c. Also, on the piezoelectric substrate 171, thereare formed a second SAW resonator of energy strage type constituted byan IDT electrode 173a and reflector electrodes 173b, 173c and a thirdSAW resonator of energy strage type constituted by an IDT electrode 174aand reflector electrodes 174b, 174c. And, these three SAW resonators aredisposed in close relations to one another, and the bus bar electrodesof mutually adjacent IDT electrodes are electrically independent. Also,the reflector electrodes are connected by the common bus bar. Theelectrode finger of the IDT electrode 172a in the first SAW resonator isconnected to the balanced type input terminal IN, and the electrodefinger of the IDT electrode 174a in the third SAW resonator is connectedto the balanced type output terminal OUT. The electrode fingers of theIDT electrode 173a in the second SAW resonator are all grounded.Furthermore, when the electrode finger crossing difference width of theIDT electrodes 172a and 174a in the first and third SAW resonator isassumed to be W1, and the electrode finger crossing difference width ofthe IDT electrode 173a in the second SAW resonator is assumed to be W2,setting is so made that the relative sizes of W1 and W2 become: W1≦W2.

With respect to the SAW filter having the above constitution, theelectrode structure of the second SAW resonator at the central part ischanged from the periodic structure strip line electrode rows in theabove third embodiment to the IDT electrode 173a, but as thetransmission of the SAW is carried out in exactly the same manner, thebasic operation is same as the case of the third embodiment shown inFIG. 10. Accordingly, flattening of passing characteristic of SAW filterand inhibition of spurious in the rejection band are realized in thesame manner as in the third embodiment.

According to this embodiment 4, three SAW resonators are disposed inadjacent relations with one another, and all the IDT electrodes 173aconstituting the central second SAW resonator are grounded, and theircrossing widths are made slightly longer than the crossing width of theIDT electrode fingers of the first and the third SAW resonators, bywhich there can be obtained a SAW filter having wide bandwidth and flatpass characteristic and acute attenuation characteristic. Furthermore,due to the electrical isolation of the bus bar at the central part ofthe IDT electrode, it becomes possible to wire the IDT electrode 172a ofthe first SAW resonator and the SAW resonator 174a of the second SAWresonator all independently, so that the balanced input and output ofthe SAW filter can be realized. Consequently, the filter characteristicbecomes free from the effect of the floating capacity or the likedepending on the grounding condition of the electrode, and thecharacteristics of the rejection band and transition band are improved.In addition, it becomes possible to connect the balanced type elementssuch as IC to the front and rear stages of the filter without using anyexternal extra circuit such as Balun, thus improving the noisecharacteristics of the whole circuit.

Furthermore, in this embodiment 4, when a plurality of SAW filters arevertically connected to constitute a multi-stage connection SAW filter,the characteristics of the transition band and the rejection band areremarkably improved. The method of vertical connection and method ofconnecting the reactance element (matching element) to the inter-stagepart are exactly the same as those of the third embodiment shown in FIG.16, and the effect on the filter characteristic is same as thatdescribed in the third embodiment.

In the above third embodiment, as shown in FIG. 10, the IDT electrode102a of the first SAW resonator and the IDT electrode 104a of the secondSAW resonator are disposed to be in reverse phase to each other.However, the invention is not necessarily limited to this constitutionbut the electrode dispositions may be of the same phase. Even in thiscase, except the slight difference in the mode of presence of extra-bandspurious, the action and effect make no difference. In this respect, thesame thing applies to the fourth embodiment.

In the above third and fourth embodiments, the input and outputterminals are of balanced type, but they are not necessarily limited tothe said constitution but it is possible to ground the unilateral sidesof the input and output terminals respectively to adopt an unbalancedtype. Moreover, in case of the grounding of either one side, a SAWfilter having balanced-unbalanced terminals can be constituted.

(Embodiment 5)

FIG. 18 shows a constitution view of an electrode pattern according toEmbodiment 5 of the SAW filter of the present invention.

In FIG. 18, the part 181 is a single crystal piezoelectric substrate. Byforming an electrode pattern of periodic structure on the piezoelectricsubstrate 181, a SAW can be excited. On the piezoelectric substrate 181,there is formed a first SAW resonator of energy strage type constitutedby an IDT electrode 182a and reflector electrodes 182b, 182c. Also, onthe piezoelectric substrate 181, there is formed a second SAW resonatorof energy strage type constituted by an IDT electrode 183a and reflectorelectrodes 183b, 183c.

As shown in FIG. 18, the IDT electrode 183a which constitutes the secondSAW resonator is constituted by the connection of the three groups ofthe first, second, and third divisional IDT electrodes 184a, 184b and184c. Here, the first divisional IDT electrode 184a and the seconddivisional IDT electrode 184b are disposed in reverse phases, and thesecond divisional IDT electrode 184b and the third divisional IDTelectrode 184c are disposed in the same phase. With respect to the samephase and reverse phase, description will be given later.

The connection methods for these three groups are as noted below.

The lower electrode (outside bus bar electrode) 1841o of the firstdivisional IDT electrode 184a and the upper electrode (inside bus barelectrode) 1842i of the second divisional IDT electrode 184b aremutually connected through the fifth electrode finger 184a5 included inthe first divisional IDT electrode 184a and a short connecting electrode184ab. Also, the lower electrode (outside bus bar electrode) 1842o ofthe second divisional IDT electrode 184b and the lower electrode(outside bus bar electrode) 1843o of the third divisional IDT electrode84c are connected.

By the above, an IDT electrode 183a which constitutes the second SAWresonator is formed.

The above grouping method is based on the divisional condition of theinside bus bar electrode and the divisional condition of the outside busbar electrode.

Namely, due to the division of the upper electrode 1843i and the upperelectrode 1842i, division is made to the third divisional IDT electrode184c and the second divisional IDT electrode 184b. Also, due to thedivision of the lower electrode 1942o and the lower electrode 1841o,division is made to the second divisional IDT electrode 184b and thefirst divisional IDT electrode 184a.

And, these two first and second SAW resonators are disposed in adjacentrelations with each other, and by the formation of acoustic couplebetween them an SAW filter is constituted.

Furthermore, the upper electrode and lower electrode of the IDTelectrode 182a are connected respectively to the balanced type inputterminal IN. The lower electrode of the first divisional IDT electrode184a and the upper electrode of the second divisional IDT electrode 184bwhich constitute the IDT electrode 183a are connected to one of thebalanced type output terminal OUT, and the lower electrode of the seconddivisional IDT electrode 184b and the lower electrode of the thirddivisional IDT electrode 184c are connected to the other of the balancedtype output terminal OUT, and the upper electrode of the firstdivisional IDT electrode 184a and the upper electrode of the thirddivisional IDT electrode 184c are grounded, by which a balanced typeinput and output terminal is formed.

Here, explanation is given on the same phase and reverse phase asdescribed above.

First, structural disposition relations of adjacent two electrodefingers (a pair of adjacent electrode fingers) are described.

That the adjacent two electrode fingers are in the same phase relationsmeans that they are in such connection relations that one of the saidtwo electrode fingers is connected to the inside bus bar electrode andextends outward from inside, and the other is connected to the outsidebus bar electrode and extends inward from outside. Also, the adjacenttwo electrode fingers are in reverse phase relations means suchconnection relations that both of said two electrode fingers areconnected to the inside bus bar electrodes and extend outward frominside, or that they are connected to the outside bus bar electrode andextend inward from outside. Here, it is assumed that the electriccharges of the inside and outside bus bar electrodes are different, andthat the pitch (distance between centers) between said adjacent twoelectrode fingers is 1/2×λ (wherein λ is wavelength of excited surfaceacoustic wave). The pitch between the electrode fingers may be(m+1/2)×λ(m=0,1,2,3. . . ). If, in such case, the pitch is (m+1)×λ, thenthe contents of meaning fully reverse with respect to the above samephase relation and reverse phase relation.

Concretely, when observed with the first divisional IDT electrode 184a,as shown in FIG. 18, for example, the first electrode finger 184a1 andthe second electrode finger 184a2 are in the same phase relation, andthe fourth electrode finger 184a4 and the fifth electrode finger 184a5are also in the same phase relation, and accordingly, all electrodefingers included in the first divisional IDT electrode 184a are in thesame phase relations. Similarly, all electrode fingers included in thesecond and third divisional IDT electrodes 184b, 184c are in the samephase relations.

Next, with respect to the pair of electrode fingers 184a5 and 184b1,because the electrode finger 184a5 is connected to the outside bus barelectrode 1841o and the electrode finger 184b1 to the outside bus barelectrode 1842o, they are in the reverse phase relations. These adjacenttwo electrodes are disposed at the separating point between the firstdivisional IDT electrode 184a and the second IDT electrode 184b.

Accordingly, needless to say, the reverse phase or same phase referredto in respect to the above disposition of the three groups is based onthe relations of the adjacent two electrode fingers as described above.This point is the same in other embodiments.

In addition, the width in the short length direction of the fifthelectrode finger 184a5 will be related to below.

In FIG. 18, the constitution in which the width of the fifth electrodefinger 184a5 is the same as that of other electrode finger is shown.However, without being limited to it, the width may of course be widerthan that of other electrode finger. By so providing, the resistancevalue of the electrode finger is lowered, and accordingly the resistancevalue of the IDT electrode containing it becomes small to give an effectof decrease in insertion loss. This applies to the case of otherembodiments.

With respect to the SAW filter in the fifth embodiment constituted asabove, the operation is explained below.

FIG. 19 is a capacity equivalent circuit diagram according to the fifthembodiment, wherein C₁ is a capacity of the IDT electrode 182a whichconstitutes the first SAW resonator. Ca, Cb and Cc are the capacities ofthe first, second, and third divisional IDT electrodes 184a, 184b, and184c, and the synthesized capacity of Ca, Cb and Cc becomes the totalcapacity C₂ of the second SAW resonator IDT electrode 183a. Here,assuming the number of couples of the electrode fingers included in theIDT electrode 183a to be n, and the respective number of couples of thethird divisional IDT electrodes 184a, 184b, and 184c to be na, nab, andnc, the relation can be expressed by the following equation:

    n=na+nb+nc

In the SAW filter as described above, the capacities of the IDTelectrodes 182a, 183a are dominated by the number of couples of theelectrode. Assuming the number of couples of the IDT electrode 182a tobe n, and the electrode capacity of a couple of IDT electrode fingers tobe C, the values of C₁, Ca, Cb and Cc can be expressed, respectively, asfollows:

    C.sub.1 =n×C

    Ca=na×C=C.sub.1 ×na/n=C.sub.1 ×na/(na+nb+nc)

    Cb=nb×C=C.sub.1 ×nb/n=C.sub.1 ×nb/(na+nb+nc)

    Cc=nc×C=C.sub.1 ×nc/n=C.sub.1 ×nc/(na+nb+nc)

Accordingly, from the capacity equivalent circuit diagram of FIG. 19,the total capacity C₂ can be expressed by the Expression 1, by using Ca,Cb, and Cc. ##EQU1##

For example, assuming that the number of couples of the divisional IDTelectrodes 184a, 184b, and 184c are equal, i.e., na=nb=nc=n/3, therelation becomes C₂ =C₁ ×1/2, and the capacity of C₂ becomes half ofthat of C₁. By changing the number of couples na, nb, and nc of thedivisional IDT electrodes 184a, 184b, and 184c, the total capacity C₂ ofthe IDT electrode 183a varies according to Expression 1 in the range ofC₁ ×1/4<C₂ <C₁. Namely, the total capacity of the IDT electrode 183a canbe controlled by the divisional ratio of the divisional IDT electrodes184a, 184b, and 184c.

Also, in this case, the electric charges on the electrodes of the first,second, and third divisional IDT electrodes 184a, 184b, and 184c are notmutually cancelled, and the SAWs formed by the first, second, and thirddivisional IDT electrods 184a, 184b, and 184c become the same phase. Sothat the second SAW resonator has the equivalent resonancecharacteristics to those of the first SAW resonator. Accordingly, bydisposing the first SAW resonator and the second SAW resonator near toeach other, they operate as the lateral mode combined resonance typefilters in the same manner as in the conventional system.

As described above, according to the present Embodiment 5, the SAWfilter having balanced type input and output shows excellentcharacteristics in the extra-band selectivity with narrow bandwidth, andalso it can control the output impedance of the SAW filter by theelectrode structure of IDT electrode which is formed by the divisionalIDT electrode which is characterized by the present invention.

In the fifth embodiment, description has been made on the IDT electrode183a which constitutes the second SAW resonator, relating to the casewhere the first, second, and third divisional IDT electrodes 184a, 184b,and 184c which constitute the IDT electrode 183a are laid from left sideto right side in order in the drawing, but the laying order may not belimited to the above but be from right side to left side as 184a, 184b,and 184c. The electrode pattern of the IDT electrode 183a may beinverted upside down. In such a case, as shown in FIG. 20, the IDTelectrode 203a which constitutes the second SAW resonator on thepiezoelectric substrate 201 is constituted by the connection of thethree groups of first, second and third divisional IDT electrodes 204a,204b and 204c. The first divisional IDT electrode 204a and the seconddivisional IDT electrode 104b are disposed in reverse mode, and thesecond divisional electrode 204b and the third divisional IDT electrode204c are disposed in the same phase, the upper electrode of the firstdivisional IDT electrode 1204a and the lower electrode of the seconddivisional IDT electrode 204b are connected, and the upper electrode ofthe second divisional IDT electrode 204b and the upper electrode of thethird divisional IDT electrode 204c are connected to form an IDTelectrode 203a which constitute the second SAW resonator. Also, in FIG.20, the divisional IDT electrodes 204a, 204b, and 204c are laid in orderof 204a, 204b, and 204c from the left, but the order may be from theright. In these cases, the difference in IDT electrodes lies only in theelectrode structures, and in respect to the characteristics of the SAWfilter, the same effect as in the case of FIG. 18 is obtainable.

In Embodiment 5, the number of couples of the IDT electrode 182a and thetotal of the number of couples of the first, second and third divisionalIDT electrodes 184a, 184b, and 184c, respectively, are equal. However,they need not be exactly same number of couples, and the ratio of thenumber of couples of the first, second and third divisional IDTelectrodes 184a, 184b, and 184c can be optionally set. Further, thenumber of division of the IDT electrode 183a is set to be 3, but thenumber may be other than that number. Furthermore, though the electricterminal for the ID electrode 182a is exemplified to be of balancedtype, either one of the upper electrode or the lower electrode may begrounded to make unbalanced electric terminal. In such a case, a SAWfilter having balanced-unbalanced terminals can be constituted. Therehas been adopted a constitution wherein the reflector electrodes 182band 183b, and 182c and 183c are electrically separated, but the twomembers may be connected and grounded. Furthermore, though it isdesigned for the IDT electrode 183a constituted by the divisional IDTelectrode 184a, 184b and 184c to constitute the second SAW resonator, itmay constitute a first SAW resonator, or both of them, and in such acase there can be realized a SAW filter capable of controlling theimpedance of both input and output sides.

(Embodiment 6)

FIG. 21 shows a constitution view of an electric pattern of SAW filteraccording to Embodiment 6 of the present invention.

In FIG. 21, the part 211 is a single crystal piezoelectric substrate. Byconstituting a periodic structure strip line form electrode pattern onsaid piezoelectric substrate 211, SAW can be excited. On thepiezoelectric substrate 211 there is formed a first SAW resonator ofenergy strage type constituted by an IDT electrode 212a and reflectorelectrodes 212b, 212c. Also, on the piezoelectric substrate 211 there isformed a second SAW resonator of energy strage type constituted by anIDT electrode 213a and reflector electrodes 213b, 213c.

The IDT electrode 213a which constitutes the second SAW resonator isconstituted by the connection of the three groups of first, second andthird divisional IDT electrodes 214a, 214b and 214c. The first, secondand third divisional IDT electrodes 214a, 214b and 214c are all disposedin the same phase, and the upper electrode of the first divisional IDTelectrode 214a and the upper electrode of the second divisional IDTelectrode 214b are connected, and by the connection of the lowerelectrode of the second divisional IDT electrode 214b and the lowerelectrode of the third divisional IDT electrode 214c, an IDT electrode213a which constitutes the second SAW resonator is formed. And, as thesetwo first and second SAW resonators are disposed in nearby relations andacoustic couple is formed therebetween, a SAW filter is constituted.

Furthermore, the upper electrode and lower electrode of the IDTelectrode 212a are respectively connected to the balanced type inputterminals IN. Also, the upper electrode of the first divisional IDTelectrode 214a and the upper electrode of the second divisional IDTelectrode 214b which constitute the IDT electrode 213a are connected toone side of the balanced type output terminal OUT, and the lowerelectrode of the second divisional IDT electrode 214b and the lowerelectrode of the third divisional IDT electrode 214c are connected tothe other side of the balanced type output terminal OUT, and the lowerelectrode of the first divisional IDT electrode 214a and the upperelectrode of the third divisional IDT electrode 214c are grounded toform the balanced type input and output terminals.

In the SAW filter constituted as above, the first SAW resonator has thesame construction as that of the SAW resonator of the fifth embodiment,and the second SAW resonator is different from that of the fifthembodiment only in respect of the electrode pattern and its connectionmethod of the IDT electrode 213a of the former from that of the IDT 183aof the latter. Even in this case, the electric charges on the divisionalIDT electrodes 214a, 214b, and 214c are not mutually canceled but theSAWs formed by the divisional IDT electrodes 214a, 214b, and 214c are ofthe same phase, and the second SAW resonator has the same resonancecharacteristics as the first SAW resonator. Therefore, by disposing thefirst SAW resonator and the second SAW resonator nearby to each other,the SAW filter of this Embodiment 6 operates as a conventional lateralmode combined resonator type filter, in the same manner as in Embodiment5. Additionally, the SAW filter having balanced type input and outputpossesses excellent characteristics of extra-band selectivity withnarrow band, and can control the input and output impedance of SAWfilter, thus giving the same effect as the SAW filter of the fifthembodiment.

In the sixth embodiment, the divisional IDT electrodes 214a, 214b, and214c are designated as 214a, 214b, and 214c from the left side, but thissequence may be taken from the right side. Alternatively, the divisionalnumber of IDT electrode 213a which is given as 3 may be set to any othernumber. The electric terminal of IDT electrode 212a which is exemplifiedas being of balanced type may be changed to unbalanced type electricterminal by grounding either the upper electrode or the lower electrode.In such a case, a SAW filter having balanced-unbalanced terminals can beconstituted. Although the constitution is such that the reflectorelectrodes 212b and 213b, and 212c and 213c are electrically separated,the two members may be connected and grounded. Furthermore, though it isdefined that the IDT electrode 213a constituted by the divisional IDTelectrode 214a, 214b and 214c is to constitute the second SAW resonator,this may constitute a first SAW resonator, or both the first and secondSAW resonators. In the latter case, a SAW filter capable of controllingthe impedance's of both input and output can be realized.

(Embodiment 7)

In Embodiments 5 and 6, explanation has been given on the case of SAWfilter of single stage constitution taken as examples. Such SAW filtersmay be used in multi-stage constitution.

FIG. 22 is an example of multi-stage constitution showing an electrodepattern constitution view of SAW filter according to Embodiment 7 of thepresent invention. In FIG. 22, the part 221 shows a single crystalpiezoelectric substrate. When a plurality of SAW filters are verticallyconnected on the piezoelectric substrate 221 to constitute a multi-stageconnection SAW filter, remarkable improvements are obtainable in thecharacteristics of rejection band and transition band, though someincrease in the insertion loss occurs.

The two-stage vertically connected filter in FIG. 22 comprises a firstSAW filter comprising a first SAW resonator constituted by an IDTelectrode 222a and reflector electrodes 222b, 222c and a second SAWresonator constituted by an IDT electrode 223a and reflector electrodes223b, 223c, which are disposed near to each other, and a second SAWfilter comprising a third SAW resonator constituted by an IDT electrode224a and reflector electrodes 224b, 224c and a fourth SAW resonatorconstituted by an IDT electrode 225a and reflector electrodes 225b,225c, which are disposed near to each other, being formed on thepiezoelectric substrate 221. The IDT electrode 225a constituting thefourth SAW resonator in the second SAW filter is composed by connectingthe three groups of the first, second, and third divisional IDTelectrodes 226a, 226b and 226c. The first divisional IDT electrode 226aand the second divisional IDT electrode 226b are disposed in reversephase, and the second divisional IDT electrode 226b and the thirddivisional IDT electrode 226c are disposed in same phase. Then, thelower electrode of the first divisional IDT electrode 226a and the upperelectrode of the second divisional IDT electrode 226b are mutuallyconnected, and the lower electrode of the second divisional IDTelectrode 226b and the lower electrode of the third divisional IDTelectrode 226c are connected, by which an IDT electrode 225a whichconstitutes the fourth SAW resonator is formed. One of the leading outelectrodes on the output side of the first stage SAW filter is connectedto the opposite leading out electrode on the input side of the oppositenext stage SAW filter by an inter-stage connecting electrode pattern227a, and another first stage IDT electrode on the output side isconnected to another next stage IDT electrode on the input side by aninter-stage connecting electrode pattern 227b, by which a two-stage SAWfilter is formed.

Furthermore, the upper electrode and the lower electrode of the IDTelectrode 222a which constitutes the first SAW resonator in the firstSAW filter are connected respectively to the balanced type inputterminal IN. Also, in the IDT electrode 225a which constitutes thefourth SAW resonator in the second SAW filter, the lower electrode ofthe first divisional IDT electrode 226a and the upper electrode of thesecond divisional IDT electrode 226b are connected to one side of thebalanced type output terminal OUT, the lower electrode of the seconddivisional IDT electrode 226b and the lower electrode of the thirddivisional IDT electrode 225c are connected to the other side of thebalanced type output terminal OUT, and the upper electrode of the firstdivisional IDT electrode 226a and the upper electrode of the thirddivisional IDT electrode 226c are grounded to form a balanced type inputand output terminal.

However, there may be cases where the purported good transmissioncharacteristics cannot be obtained by a simple vertical connection ofthe SAW filters, due to the non-matching of the input and outputimpedance's of stages. In such a case, a reactance element such asinductor may be connected as a matching element to the inter-stageconnection electrode to make adjustment. Alternatively, there may beadopted such a constitution as to form a reactance element representedby a spiral inductor on the same piezoelectric substrate 221 or on aseparate substrate and connect it to the inter-stage connectionelectrode, by which size reduction of the filter circuit can be easilyrealized without requiring extra space. With respect to the reactanceelement for adjustment, connection may be made to either one of thefirst inter-stage connection electrode pattern 227a or 227b, and otherinter-stage electrode connecting pattern may be grounded. According tothe experiment, as shown in FIG. 22, connection of the reactance element228 to the inter-stage connection electrode pattern 227a proved to giveimprovement to the symmetry of filter transmission characteristics.

By the above constitution, the SAW filter having balanced type input andoutput in this Embodiment 7 shows narrow band characteristics, and byconnecting two SAW filters by inter-stage connection electrode patterns227a, 227b, the extra-band selectivity comes to show more acutecharacteristic than in the case of a single stage, and also it becomespossible to control the output impedance of the SAW filter.

In the seventh embodiment, in the IDT electrode 225a constituting thefourth SAW resonator in the second SAW filter, the first, second andthird divisional IDT electrodes 226a, 226b, and 226c which constitutethe IDT electrode 225a are designated as 226a, 226b, and 226c from theleft side facing the drawing, but this sequence may be taken from theright side. The electrode pattern of the IDT electrode 225a may bereversed upside down.

In this Embodiment 7, the divisional number of IDT electrode 225a isgiven as 3, but it may be set to any other number. The electric terminalof IDT electrode 222a which is exemplified as being of balanced type maybe changed to unbalanced type electric terminal by grounding either theupper electrode or the lower electrode. In such a case, a SAW filterhaving balanced-unbalanced terminals can be constituted. The IDTelectrode 225a may be an IDT electrode 213a shown in Embodiment 6. Inthese cases, the IDT electrode 234a is different only in electrodeconstitution, and as to the SAW filter characteristic, the same effectas in FIG. 22 can be obtained. Though there is adopted such constitutionthat the reflector electrodes 222b and 223b, and 222c and 223c areelectrically separated, the two members may be connected and grounded.Furthermore, though it is defined that the IDT electrode 225aconstituted by the divisional IDT electrode 226a, 226b and 226c is toconstitute the fourth SAW resonator, this may constitute a first SAWresonator, or both the first and fourth SAW resonators. In the lattercase, a SAW filter capable of controlling the impedance's of both inputand output can be realized. Also, the number of stages of SAW is shownas two stages, but the number may be larger, in which case the filtercharacteristics are acute, with more excellent extra-band selectivity.

(Embodiment 8)

FIG. 23 shows a constitution view of an electrode pattern of SAW filteraccording to Embodiment 8 of the present invention. In FIG. 23, the part231 is a single crystal piezoelectric substrate. By forming an electrodepattern on said piezoelectric substrate 231, SAW can be excited. On thepiezoelectric substrate 231 there is formed a first SAW resonator ofenergy strage type constituted by an IDT electrode 232a and reflectorelectrodes 232b, 232c. Also, on the piezoelectric substrate 231 there isformed a third SAW resonator constituted by an IDT electrode 234a andreflector electrodes 234b, 234c. The electrode part 233a of the secondSAW resonator formed between the first SAW resonator and the third SAWresonator, accompanied with reflector electrodes 233b, 233c, has thesame construction as the reflector electrode. In this way, even when thestructure of the electrode part 223a of the second SAW resonator is notthe IDT electrode structure but a periodic structure strip lineelectrode row, if the electrode period is the same, SAW can bepropagated in exactly the same manner, so that the acoustic behaviors ofthe second SAW resonator disposed at the central part make no differencefrom those of the case of IDT electrode structure.

Furthermore, the IDT electrode 234a which constitute the third SAW isconstituted by the connection of the three groups of first, second andthird divisional IDT electrodes 235a, 235b and 235c. The firstdivisional IDT electrode 235a and the second divisional IDT electrode235b are disposed in reverse phases; the second divisional IDT electrode235b and the third divisional IDT electrode 235c are disposed in thesame phase; the lower electrode of the first divisional IDT electrode235a and the upper electrode of the second divisional IDT electrode 235bare connected; and the lower electrode of the second divisional IDTelectrode 235b and the lower electrode of the third divisional IDTelectrode 235c are connected to form an IDT electrode 234a whichconstitutes the third SAW resonator.

The above three SAW resonators are disposed in nearby relation with oneanother, and the bus bar electrodes of the mutually adjacent parts areelectrically independent. The upper electrode and the lower electrode ofIDT electrode 232a which constitute the first SAW resonator in the firstSAW filter are connected respectively to the balanced type inputterminal IN. Also, in the IDT electrode 234a which constitutes the thirdSAW resonator, the lower electrode of the first divisional IDT electrode235a which constitutes the IDT electrode 234a and the upper electrode ofthe second divisional IDT electrode 235b are connected to one side ofthe balanced type output terminal OUT, and the lower electrode of thesecond divisional IDT electrode 235b and the lower electrode of thethird divisional IDT electrode 235c are connected to the other side ofthe balanced type output terminal OUT, and the upper electrode of thefirst divisional IDT electrode 235a and the upper electrode of the thirddivisional IDT electrode 235c are grounded to form a balanced type inputand output terminal, and the periodic structured strip line electrodeline 233a in the second SAW resonator is grounded.

As described above, the SAW filter according to this Embodiment 8 ischaracterized by realizing a filter characteristic by disposing thethree SAW resonators nearby in parallel with the direction ofpropagation of the SAW to make acoustic couple.

At this time, the SAW filter is a substitution of the IDT electrode 233awhich constitutes the second SAW resonator in the SAW filter of thepresent invention for the IDT electrode in the SAW multi-mode filter ofJapanese Patent Kokai Publication No. 8-51334 published by the presentinventors, and it shows the same operation as that described in saidPublication No. 8-51334. Namely, by making the SAW resonator in threestages, the filter can have wide band width, and characteristicsexcellent in extra-band selectivity, and also can control the outputimpedance of the SAW filter.

In the eighth embodiment, in the IDT electrode 234a constituting thethird SAW resonator, the first, second and third divisional IDTelectrodes 235a, 235b, and 235c which constitute the IDT electrode 234aare designated as 235a, 235b, and 235c from the left side facing thedrawing, but this sequence may be taken from the right side. Theelectrode pattern of the IDT electrode 234a may be reversed upside down.The IDT electrode 234a may be the IDT electrode 213a of the constitutionshown in Embodiment 6. In these cases, the IDT electrode 234a isdifferent only in electrode constitution, and as to the SAW filtercharacteristic, the same effect as in FIG. 23 can be obtained.

Also, the divisional number of IDT electrode 234a is given as 3, but itmay be set to any other number. The electric terminal of IDT electrode232a which is exemplified as being of balanced type may be changed tounbalanced type electric terminal by grounding either the upperelectrode or the lower electrode. In such a case, a SAW filter havingbalanced-unbalanced terminals can be constituted. Though there isadopted such constitution that the reflector electrodes 232b and 233b,and 232c and 233c are electrically separated, the two members may beconnected and grounded. Furthermore, though it is defined that the IDTelectrode 234a constituted by the divisional IDT electrode 235a, 235band 235c is to constitute the third SAW resonator, this may constitute afirst SAW resonator, or both the first and third SAW resonators. In thelatter case, a SAW filter capable of controlling the impedance's of bothinput and output can be realized.

In this Embodiment 8, the IDT electrode 233a is described as beinggrounded through the electrode pattern provided in the space between theIDT electrode 232a and the reflector electrode 233c on the right sidethereof. However, it may be grounded through the electrode patternprovided in the space between the IDT electrode 233a and the reflectorelectrode 233a on the left side thereof, or alternatively it may begrounded through the electrode pattern provided in the space between theIDT electrode 234a and either one of the reflector electrode 234b or234c. Though there is adopted such constitution that the reflectorelectrodes 232b and 233b, and 232c and 233c are electrically separatedon each SAW resonator, they may be respectively connected and grounded.Furthermore, the IDT electrode 233a may be grounded through any of thereflector electrodes 232b, 232c, 233b, 233c, 234b , and 234c. The IDTelectrode 233a may be of the electrode structure of the sameconstitution as that of the IDT electrode 232a. In this case also,propagation of SAW is performed in the same manner, and the similarcharacteristic as that of the SAW filter of this Embodiment 8 isobtainable. Furthermore, though it is described that the divisional IDTelectrode 234a is to constitute a third SAW resonator, it may beconstituted by a first SAW resonator, or both of them. In the lattercase, a SAW filter capable of controlling the impedance's of both inputand output can be realized. Although the first to the third SAWresonators are shown to be of the same constitution, they need notnecessarily be the same. The SAW filters of Embodiment 8 may be of twostage vertical connection, in which case the extra-band selectivitycharacteristic becomes further acute.

As to the piezoelectric substrate in the present invention, use of an STcut crystal having excellent temperature characteristics is preferable,but there may be used as substrates LiTaO₃, LiNbO₃, Li₂ B₄ O₇, La₃ Ga₃SiO₁₄ and the like. As an electrode material, use of relatively lowdensity aluminum whose film thickness control is easy is preferable, butuse of gold electrode is also possible.

Furthermore, the present invention is applicable to resonators using notonly the SAW described above but also SSBW(Surface Skimming Balk Wave)which is one of the SAW or Pseudo surface waves, and the like.

What is claimed is:
 1. A surface acoustic wave filter on a piezoelectricsubstrate comprising first and second surface acoustic wave resonatorseach having a reflector electrode on both sides of an IDT electrode asan inter-digital transducer electrode, said resonators being disposednearby in positions in which directions of propagation of respectivesurface acoustic waves are parallel with each other and acousticallycoupled,an inside bus bar electrode included in the first IDT electrodeof the first surface acoustic wave resonator and an inside bus barelectrode included in the second IDT electrode of the second surfaceacoustic wave resonator being mutually electrically separated, saidfirst IDT electrode being connected to a balanced type input terminal,and said second IDT electrode being connected to a balanced type outputterminal, one terminal of said balanced type input terminal beingelectrically connected to leading out electrodes led out directly orindirectly from at least two places of the inside bus bar electrode ofsaid first IDT electrode, and one terminal of said balanced type outputterminal being electrically connected to leading out electrodes led outdirectly or indirectly from at least two places of the inside bus barelectrode of said second IDT electrode, thereby performing balancedoperation.
 2. A surface acoustic wave filter according to claim 1,wherein the area between the two leading out electrodes each formed in aspace between the IDT electrode and said reflector electrode isconnected by a wiring pattern having a wider line width than the widthof said leading out electrodes formed on said piezoelectric substrate,afurther expanded position in said wiring pattern is a connection land ofone terminal of said balanced type input terminal or as one terminal ofsaid balanced type output terminal, and a position of extension in anoutward direction of the outside bus bar electrode included in said IDTelectrode is the connection land of the other terminal of said balancedtype input terminal or of the other terminal of said balanced typeoutput terminal.
 3. A surface acoustic wave filter according to claim 1,wherein two leading out electrodes formed in a space between the IDTelectrode and said reflector electrode are connected to make oneterminal of said balanced type input terminal or one terminal of saidbalanced type output terminal, and to make the outside bus bar electrodeincluded in said IDT electrode the other terminal of said balanced typeinput terminal or said balanced type output terminal.
 4. A surfaceacoustic wave filter according to claim 1, wherein at least one leadingout electrode is formed in a space between the IDT electrode and saidreflector electrode.
 5. A multi-stage surface acoustic wave filtercomprising a plurality of stages of the surface acoustic wave filters ofclaim 4 formed on a same piezoelectric substrate,one of the leading outelectrodes on the output side of the front stage surface acoustic wavefilter being connected to the opposed leading out electrode on the inputside of the next stage, the other of the leading out electrodes on theoutput side of the front stage surface acoustic wave filter beingconnected to the opposed leading out electrode on the input side of thenext stage, and a remaining one output side electrode of said frontstage surface acoustic wave filter being connected to a remaining oneinput side electrode of said next stage surface acoustic wave filter. 6.A multi-stage surface acoustic wave filter comprising a plurality ofstages of the surface acoustic wave filters of claim 4 formed on a samepiezoelectric substrate,one of the leading out electrodes on the outputside of the front stage surface acoustic wave filter and the opposedleading out electrode on the input side of the next stage, and the otherof the leading out electrodes on the output side of the front stagesurface acoustic wave filter and the opposed leading out electrode onthe input side of the next stage, being connected respectively by firstinter-stage connecting electrodes having a wider width than a width ofsaid leading out electrodes, a remaining one output side electrode ofsaid front stage surface acoustic wave filter and a remaining one inputside electrode of said next stage surface acoustic wave filter beingconnected respectively by a second inter-stage connecting electrodehaving a wider width than the width of said leading out electrodes, aspace between the two leading out electrodes on the input side of thefirst stage surface acoustic wave filter being connected by a wiringpattern having a line path width wider than the width of said leadingout electrodes formed on said piezoelectric substrate, a furtherexpanded part in said wiring pattern being a connecting land as oneterminal of said balanced type input terminals, and an outwardlyexpanded part of said outside bus bar electrode included in said IDTelectrode of said first stage surface acoustic wave filter being aconnecting land as the other terminal of said balanced type inputterminals, and a space between the two leading out electrodes on theoutput side of the last stage surface acoustic wave filter beingconnected by a wiring pattern having a line path width wider than thewidth of said leading out electrodes formed on said piezoelectricsubstrate, a further expanded part in said wiring pattern being aconnecting land as one terminal of said balanced type output terminals,and an outwardly expanded part of said outside bus bar electrodeincluded in said IDT electrode of said last stage surface acoustic wavefilter being a connecting land as the other terminal of said balancedtype output terminals.
 7. A multi-stage surface acoustic wave filteraccording to claim 6, wherein the space between the first and secondinter-stage connection electrodes is connected through a reactanceelement.
 8. A multi-stage surface acoustic wave filter according toclaim 6, wherein, of the first and second inter-stage connectionelectrodes, one is grounded and the other is grounded through areactance element.
 9. A multi-stage surface acoustic wave filteraccording to claim 6, wherein said first inter-stage connectionelectrode is grounded through a reactance element, and the secondinter-stage connection electrode is grounded.
 10. A surface acousticwave filter on a piezoelectric substrate comprising:a first surfaceacoustic wave resonator having reflector electrodes on both sides of afirst IDT electrode for exciting a surface acoustic waves, and a secondsurface acoustic wave resonator having reflector electrodes on bothsides of a second IDT electrode, said first and second acoustic waveresonators being disposed nearby to each other in positions in whichpropagation directions of respective surface acoustic waves becomeparallel and acoustically coupled, wherein an inside first bus barelectrode is included in said first IDT electrode and an inside secondbus bar electrode is included in said second IDT electrode, said firstand second bus bar electrodes being mutually separated and disposed inopposed manner, one input terminal of balanced type input terminals isconstructed by using an electrical connection between leading outelectrodes led out from at least two places on said inside first bus barelectrode, and one output terminal of balanced type output terminals isconstructed by using an electrical connection between leading outelectrodes led out from at least two places on said inside second busbar electrode, thereby performing balanced operation.